The delayed coking process is a cyclic process that typically requires 18-24 hours for a complete cycle. To improve operational efficiency, delayed cokers typically operate in pairs with one coke drum filling and carrying out the coking conversion process while the other coke drum undergoes decoking operations. Thus, the first half of each cycle includes conversion of hydrocarbon feedstock and filling of the coker drum. Upon substantial completion of the conversion reaction, a steam-stripping step substantially removes trapped volatile compounds from within the coke-bed. Following steam stripping the first half of the cycle concludes with blow-down and water quenching of the coke. The second half of each cycle involves the decoking process.
In the first half of the coking operation cycle, the steam-stripping step to remove volatile compounds commonly requires about an hour. The steam stripping step injects steam from a boiler through the bottom of the coke drum upward through the coke-bed thereby transferring heat to and carrying volatile compounds out through the overhead line of the coke drum.
The conversion of heavy petroleum products to coke and additional liquid products follows well known kinetics and reaction mechanisms. During the coking conversion process, thermal cracking of the heavy hydrocarbons follows first order kinetics dominated by free radical reaction mechanisms described below.
Step—1 Thermal cracking—initiation reactions—random production of free radicalsResid or AsphaltenesArx°+Ary°+R1°+R2°
Step—2 Propagation reactions—capping of free radicals (liquid and gas formation)Arx°+Cx—HyAr—H (liquid)+free radicalsR1° or R2°+Cx—HyR1—H or R2—H (liquid)+free radicalsR1° or R2°R3═ or R4═(liquid)+H2+(C1-C4) Gaseous hydrocarbons                Where:        Length of the carbon chain in R3 & R4<R1 & R2         R3═ and R4═ are olefins        Arx° & Ary° are free radicals of poly-nuclear aromatic structures (x and y being the number of aromatic nuclei)        R1° & R2° are free radicals of various paraffinic structures        
Step—3 Termination reactions (poly-condensation of aromatic free radicals)Arx°+Ary°Coke formation